Within the radar engineering different types of microwave amplifier tubes for amplification of a radar signal are used, for example TWT (Travelling Wave Tube) or Klystron tube, which are grid pulsed tubes. TWT is mainly used within the frequency range of 1 GHZ-40 GHZ. To operate a TWT, usually, cathode voltages of -10 kV to -35 kV are required, whereby the ray speed is principally a function of the potential between the cathode and the anode. One problem is that the amplitude and the phase displacement of the emitted microwave signal are modulated through variations of the cathode voltage. When the radar pulse is emitted, the cathode current is delivered from an output capacitor C.sub.out and the voltage over it drops, which must be recharged between the pulses with a constant current. Usually, physical dimensions limit the size of the C.sub.out and maximal allowed stored energy.
"Staggered" transmission patterns are used in certain radar installations, preferably in installations of type pulse-Doppler radars, which means that the inter-pulse spacing (time between the pulses) is not constant. This means that the charge time for C.sub.out varies between different pulses, which results in varying cathode voltage. Therefore, the phase displacement of the transmitted radar signal will vary from pulse to pulse, which may have negative consequences for the ability of the system to suppress echoes from solid targets (so-called ground clutter) and results in deteriorated performance.
U.S. Pat. No. 4,682,369 discloses a solution, in which a radar transmitter includes a so-called "ripple and droop" reducing unit for decreasing the ripple and droop of the supply circuit of the transmitter. A filter capacitor for cathode supply is connected to the voltage supply and an operational amplifier (OP-amplifier) has its balanced AC-input connected across the output capacitor. An amplifier stage inverts the signal from the OP-amplifier and amplifies its absolute value to the same value that occurs across said filter capacitor. A follow-up stage, connected to the amplifier stage, receives the inverting signal and produces an output signal in series with the filter capacitor for elimination of the ripple, which output signal is supplied to a TWT connected to the supply capacitor.
In this assembly, the ripple is measure across the output capacitor of the cathode voltage supply and a mirror image of the ripple voltage is added via a series resistance to the cathode voltage. Hence, the ripple voltage is compensated. A major drawback with this solution, because of its construction, is the consumption of a relative large amount of power. Furthermore, the high voltage is connected directly to the detecting circuit, which make great demands upon the components. Moreover, the circuit lacks a closed regulation loop.
The Soviet patent document, SU 1112535 A, describes a regulation arrangement for compensating the variations in the supply voltage in a microwave transmitter. The arrangement includes a first capacitor, a second capacitor arranged in series with said first capacitor and a control circuit connected to the second capacitor. The control circuit, through a voltage divider (14), detects a supply voltage to the transmitter tube and no current is conducted through the capacitor. Thereby, the control circuit in cooperating with a power unit (20) generates a voltage across the second capacitor. Thus, the supply voltage to the tube consists of the sum of three voltages, i.e. the voltage across the power unit (20), the voltage across the first capacitor and the voltage across the second capacitor.